The prior art is replete with DC-to-DC converters, which are circuits or devices designed to produce an output DC voltage in response to an input DC voltage. For example, a DC-to-DC converter can be used to regulate the supply voltage (e.g., the collector or drain voltage) of an RF power tracking amplifier, thus resulting in more efficient operation. DC-to-DC converters may be used to vary the supply voltage of the transmit RF power amplifier of a wireless device, such as a CDMA mobile handset. In such applications, the DC-to-DC converter should have good light-load efficiency because the RF power amplifier usually operates at 15–30 dB below the maximum output power level. In mobile applications, the DC-to-DC converter should also be capable of supplying the voltage required by the RF power amplifier during maximum RF output power with little voltage drop from the battery voltage. Furthermore, the DC-to-DC converter should be able to provide a continuously changing voltage because any step change to the RF power amplifier supply voltage may cause the handset to lose its RF communication channel.
Many dynamic supply RF power amplifiers employ a buck converter to generate the desired DC output voltage. As depicted in FIG. 1, most implementations use a buck converter 100 to convert a battery voltage (identified as Vdd in FIG. 1) to a lower voltage Vout in response to an input DC voltage Vin as needed by an RF power amplifier 102. The thick arrow in FIG. 1 represents a high peak current condition. Since RF power amplifier 102 is not directly connected to the battery, buck converter 100 must be able to supply a relatively high peak current to RF power amplifier 102 to handle high power conditions. Consequently, the switching transistors 104/106 must be relatively large in size to carry such high peak current. In addition, when RF power amplifier 102 operates in a relatively low power mode, it draws a relatively low peak current, which can cause buck converter 100 to operate inefficiently. Furthermore, at maximum current output, Vout generated by buck converter 100 must be as close as possible to the battery voltage. At maximum current output, however, the DC current flows through transistor 104 and an inductor 108. Thus, in an effort to minimize voltage drop across transistor 104 and inductor 108, the “on” resistance of transistor 104 and the DC resistance of inductor 108 must be minimized, which usually requires a very large sized transistor 104 and a very large sized inductor 108, both of which are undesirable in practical embodiments.
FIG. 2 is a schematic representation of a prior art DC-to-DC converter 200 that includes a buck converter 202 connected in parallel with a bypass transistor 204. In this arrangement, bypass transistor 204 has a relatively low “on” resistance, and bypass transistor 204 is connected to a battery 206. Although not shown in FIG. 2, the DC output Vout is connected to the drain or collector of the RF power amplifier. When the RF power amplifier needs to operate at relatively high RF power levels, its drain/collector is connected to battery 206 through bypass transistor 204. At lower power levels, however, buck converter 202 (which includes a transistor 208 and an inductor 210) provides a reduced Vout to the RF power amplifier to increase the overall efficiency. Although such an implementation can employ smaller transistors for buck converter 202 (in comparison to the arrangement shown in FIG. 1), and can avoid an undesirable voltage drop through transistor 208 and inductor 210, the change of state between the buck converter mode of operation and the bypass transistor mode of operation is non-continuous. In other words, either buck converter 202 or bypass transistor 204 (but not both) is active at any given time, and DC-to-DC converter 200 merely switches between the two modes using a controller 212. In a practical deployment, the resulting step change in the supply voltage of the RF power amplifier can cause a wireless handset to lose its connection with the wireless base station.
Accordingly, it is desirable to have a DC-to-DC converter that employs relatively small sized transistors and reactive components. In addition, it is desirable to have a DC-to-DC converter that can efficiently generate low DC output voltages corresponding to relatively low peak currents, and high DC output voltages corresponding to relatively high peak currents, in a continuous manner that minimizes undesirable output voltage drops and switching losses. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.